1. Field of the Invention
This invention relates to an active sound muffler for reducing noises that are diffracted to propagate by a sound insulating wall. More particularly, this invention relates to an active sound muffler that is effective for noises in which low frequency sounds are dominant.
2. Description of the Related Art
Sound insulating walls are built along certain trunk roads loaded with heavy traffic. The known noise reducing techniques using sound insulating walls are apparently classified into two categories. One is to insulate sounds simply by building a tall sound insulating wall along a road in order to block noises. The other is to provide a noise reducing device at the top end of the sound insulating wall built along the road in order to reduce propagating noises without making the wall very high.
The techniques utilizing a sound reducing device are further divided into passive techniques and active techniques from the viewpoint of the underlying principle adopted for noise reduction. Passive techniques include the use of branching type sound insulating walls that utilize interference of sounds, glass wool cylinders, sound absorbing cylindrical edges formed by using a 20 μm thick PVF film, a perforated aluminum plate and a stainless steel grill and soft edges adapted to produce an acoustically soft surface by using an acoustic pipe that is designed optimally based on the wavelengths of noises that may be involved. These techniques are effective for medium and high pitch sounds.
On the other hand, active techniques include electrically producing a soft surface (zero sound pressure) for active sound control using loudspeakers and microphones without changing the length of the acoustic pipe. This technique is effective for low pitch sounds.
A loudspeaker used for such an active technique can be approximated to a point sound source. Generally, a popular cone type loudspeaker showing radiation characteristics of a spherical wave is used.
Known active sound mufflers using loudspeakers operating as point sound sources are accompanied by a problem as described below. The diffracted sounds are not necessarily in phase with each other in the longitudinal direction (along the road) at the top end of the sound insulating wall. Particularly, road noises that sound insulating walls are required to deal with are low frequency noises showing a frequency band as wide as hundreds of several Hz. Therefore, if the sound insulating wall has a length exceeding 1 m, it also exceeds a half wavelength of road noises and hence, generally speaking, diffracted sounds are, if partly, out of phase with each other.
This problem may be avoided by partitioning the space at the top end of the sound insulating wall so that diffracted sounds may become in phase with each other along the surface to be controlled in a sound field where they are originally out of phase in the longitudinal direction and arranging a control loudspeaker for the surface where diffracted sounds are made in phase with each other. However, if noises that are to be reduced have a frequency of 500 Hz, the space needs to be partitioned at least by every 34 cm. Then, as many control loudspeakers and microphones as the number of divisional spaces need to be installed. If, on the other hand, control loudspeakers are arranged simply for every half wavelength to control noises. There can be produced regions where sounds are boosted because the acoustic energy of diffracted sounds is not minimized at the top end of the sound insulating wall, although the sound pressure may be reduced at the positions of the control microphones.